Gas turbines are well known in the art of power generation. A gas turbine comprises a compressor section where air is pressurized. This air then flows to a plurality of radially arranged combustion chambers in which fuel is combusted to form a hot combustion gas. The hot gas passes through a transition piece into a first stage of a turbine where the enthalpy of the gas is converted into mechanical energy. It is noted that transition piece alternatively is referred to as a “tail pipe” or “transition duct” by some in the field. Prior art references that are hereby incorporated by reference, particularly for the teachings of the structure of transition pieces and for the sources of stresses thereto, are: U.S. Pat. No. 4,422,288 to Steber, issued Dec. 27, 1983; U.S. Pat. No. 5,906,093 to Coslow et al., issued May 25, 1999; U.S. Pat. No. 6,463,742 B2 to Mandai et al., issued Oct. 15, 2002; and U.S. Pat. No. 6,662,568 B2 to Shimizu et al., issued Dec. 16, 2003. Also of interest is U.S. Pat. No. 6,523,352 B1, to Takahashi et al., issued Feb. 25, 2003, incorporated by reference in its entirety.
The transition piece receives hot combustion gases. As such the transition piece and components attached thereto are subject to stress from high temperatures, vibrations, and extreme temperature gradients over long periods of operation. Some gas turbine transition pieces are cooled by forcing air over the outside of the units while other transition pieces contain cooling channels through which forced air or steam flow to cool the transition pieces. The latter types are known generally as forced-cooled transition pieces.
Forced-cooled transition pieces include steam-cooled transition pieces in which steam is supplied to the transition piece via intake (i.e., supply) tubing and in which separate exhaust tubing returns the hotter steam from the transition pieces back to a steam system. For example, one set of steam-cooling operational parameters for cooling a transition piece include: inlet (i.e., supply) steam around 500 degrees Fahrenheit (“° F.”) inlet pressure around 260 pounds per square inch (“psi”) and outlet or exhaust steam temperature around 1000° F.
Prior art piping or tubing assemblies that connect forced cooling fluid supply and return systems to a transition piece are comprised of rigid pipe that is welded at each bend. Forced air and steam are the common force-cooled fluids, and a unitary manifold is a common structure to convey supply side and return side fluids. An example of a prior art welded tubing assembly that transports steam is shown in FIG. 1. A supply tubing assembly 2 transports steam from an outlet of a steam manifold 3 to a steam inlet port 4 of the transition piece 5. A return or exhaust tubing assembly 6 carries return steam heated by passage through channels in the transition pieces 5 from the steam outlet port 7 to the return port 8 of the steam manifold 3. Although it is known in the art to provide bracing along the lengths of this welding tubing, as indicated in FIG. 1 by brace 9, this brace merely attaches a uniformly rigid welded tubing assembly to parts of the transition piece. The tubing assembly to both sides of such bracing is of the same rigid pipe and is welded, as is taught in the prior art.
Construction of such welded rigid pipe assemblies requires substantial labor. Also, if the fit between manifold and port is not accurate, and/or if there is improper handling during shipping or installation, static loading may be imposed on the tubing assembly that shortens its useful life.
Temperature stresses may arise from the sustained high temperature on a component of the tubing assembly, from exposure to a high temperature gradient along a length of material, or from both. In addition to temperature stresses the transition piece and the tubing assemblies associated with it are subject to vibrations, such as from the varying nature of the combustion, and from related vibrations transferred from the manifold. As noted above, certain stress might accrue from undesirable static loading on the assembly such as when improper handling, by the supplier and/or due to improper installation, strain one or more of the tubing assemblies or their components. As the tubing assemblies or their components having such static loading are then brought up to operational temperature, and remain there for extended operating periods, additional stress from the initial static loading can contribute to the other stresses.